The chapter discusses the design and principles of oxide semiconductor gas sensors, which utilize porous polycrystalline resistors made from semiconducting oxides such as SnO2, WO3, ZnO, and In2O3. The key functions of these sensors are the receptor function, transducer function, and utility factor. The receptor function involves the surface properties of oxide grains that adsorb or react with target gases, while the transducer function deals with the electrical resistance changes induced by gas interactions at the grain boundaries. The utility factor is influenced by the porous structure of the sensing body, which affects the gas penetration depth and response. Recent advancements in sensor design focus on selecting high-mobility conduction electron base oxides, enhancing surface reactions with foreign receptors, and fabricating highly porous, thin sensing bodies. The chapter also highlights the importance of understanding these factors for improving gas sensing properties, particularly in applications like gas leak detection and environmental monitoring.The chapter discusses the design and principles of oxide semiconductor gas sensors, which utilize porous polycrystalline resistors made from semiconducting oxides such as SnO2, WO3, ZnO, and In2O3. The key functions of these sensors are the receptor function, transducer function, and utility factor. The receptor function involves the surface properties of oxide grains that adsorb or react with target gases, while the transducer function deals with the electrical resistance changes induced by gas interactions at the grain boundaries. The utility factor is influenced by the porous structure of the sensing body, which affects the gas penetration depth and response. Recent advancements in sensor design focus on selecting high-mobility conduction electron base oxides, enhancing surface reactions with foreign receptors, and fabricating highly porous, thin sensing bodies. The chapter also highlights the importance of understanding these factors for improving gas sensing properties, particularly in applications like gas leak detection and environmental monitoring.